ONE just turned 39 years old a month ago. One will remain 39 for another couple of weeks, before hitting the mother of all birthday milestones. Thirty-nine is a hell of a place to be, as anyone who’s already gotten there can tell you. There are times, at 39, when you wake up and feel every bit of 69.

And times, at 39, when you wake up feeling 19.

Mike Mussina, a month shy of his 40th birthday, walked away from baseball yesterday, retired from the Yankees, became only the fifth man since 1903 to say good-bye after a 20-win season in the big leagues. He sounds like a man at peace with his life, with his career, and with a coming season of tranquility.

Brett Favre, a month past his 39th birthday, walked into the Jets’ training facility in Florham Park, N.J., and got himself another day closer to saying hello to the Tennessee Titans, a team that has yet to lose a football game and has yet to find a quarterback of any age it doesn’t take great pleasure in tormenting. He sounds like a man at peace with his life, with his career, and with being in the middle of another season of toil.

“I knew in January,” Mussina said yesterday, “that this was going to be my last year. I knew from spring training. And I didn’t want this season to be only about that.”

“I have people, friends, family, who ask me all the time about next year,” Favre said on Wednesday. “The answer I give them, which is the honest answer, is, ‘I really don’t know.’ ”

It is a question as old as sport itself, a debate that has raged ever since it became apparent that age and time are more cruel to elite athletes than to just about anyone else, anywhere, any time. When is enough enough? When is it right to ride off into the sunset. When is it time, as Willie Mays once famously said, to say good-bye to America?

Well, is it any wonder how confusing that question must be to the athletes themselves when it completely flummoxes those of us who do nothing more than follow their careers? We don’t want athletes to hang around too long, to be treated like a tackling dummy like Johnny Unitas, but we don’t understand how they can leave the arena with so much left to give, like Barry Sanders.

How do you leave so soon, as Michael Jordan did in 1993? And how do you stay so late, as Michael Jordan did in 2003?

“I love baseball and I love pitching,” Mussina said. “And good seasons tend to go quickly. And before I knew it, it was September.”

“Your focus is so much different than it was as a younger player,” Favre said. “You just appreciate the moment a lot more because you realize it won’t be there forever. This game could be my last. When you’re 22 years old, you’re looking for that new contract, you’re looking for commercials, you’re looking for whatever. You just always say you’re on scholarship still.”

When to stay? When to leave? If you saw Mussina pitch at all in 2008, you mostly saw the perfect marriage of thrower and thinker, an athlete who’d been able to seamlessly integrate the talent in his right arm and the intelligence in his brain. You knew those 20 wins weren’t a fluke. And yet, on those days when his stuff wasn’t there, you could understand why he might be willing to go.

If you’ve seen Favre play at all in 2008, you’ve mostly seen what football can do to even the most blessed of athletes, you’ve seen the ravages on his body and the occasional dents in his reputation, seen more than a few footballs thrown up for grabs. And yet, on nights like last Thursday in New England, you could understand why he might never want to go.

When to stay? When to leave? Brett Favre and Mike Mussina probably have very different answers to both. Depending on the day.